With thousands of runners still on the course, two bombs exploded near the finish line of the Boston Marathon on Monday, killing three people, injuring at least 113 and turning the city’s most celebrated event into a grisly spectacle of shattered glass, blood and screams.

President Barack Obama said authorities did not know who carried out the attack but vowed to render “the full weight of justice” against those responsible. Minutes later, law enforcement officials said that an 8-year-old child was one of the dead.

Video from the scene showed two blasts about 20 seconds apart just off the course at the finish. White smoke rose, barriers flew, and throngs of people who had gathered to cheer the runners turned and fled in terror. They later reported seeing horrific injuries that included blown-off limbs and bodies thrown to the asphalt.

“All the sudden there was a massive boom. There was a sort of concussive blow that pushed a lot of people back. I could see runners falling in front of me,” said Dave Abel, a reporter for The Boston Globe who was about 10 feet from one of the explosions.

“When the smoke started to clear, I could see lots of bodies,” he said. “I could see one woman staring vacantly into the sky. I could see a lot of mangled limbs, a lot of blood and shattered glass. It was probably the most horrific thing I’ve ever seen.”

Larissa Brinkley, who came from Pennsylvania to run the race, said people dropped everything and ran the opposite way. Other witnesses described what at first sounded like a cannon blast or fireworks.

“Then it went off again. And then all of a sudden we heard people crying and running away,” said Serghino Rene, who was a few blocks away. “It was a huge horde of people just running away.”

The FBI took control of the investigation through its multiagency Joint Terrorism Task Force.

"It is a criminal investigation that is a potential terrorist investigation," said FBI special agent in charge Richard DesLauriers.

As he announced the death toll had risen to three, Boston Police Commissioner Ed Davis said authorities "will turn every rock over" to find out who is responsible.

"There is no suspect," he said. "There are people we are talking to."

Earlier, federal officials told NBC News that Boston police were guarding a “possible suspect” who had been wounded in the blasts, but they cautioned that there was no information at the federal level to consider that person a suspect.

A third, undetonated device was found near the finish line, a House Homeland Security Committee official and three law enforcement officials told NBC News. Authorities also reported an explosion at the John F. Kennedy presidential library, elsewhere in the city, more than an hour after the blasts, but police said that it appeared to be caused by a fire. The police commissioner urged people to stay inside.

Massachusetts Gov. Deval Patrick said the city of Boston would be open Tuesday but warned, "It will not be business as usual." He said security would be tight and Bostonians should remain vigilant.

"We're gonna get through this," Patrick said.

Hospitals reported that at least two children were among the injured. Dr. Alisdair Conn, chief of emergency medicine at Massachusetts General Hospital, characterized the wounds as something Americans are more accustomed to seeing on the news from a military-style bombing in Iraq or Israel.

“We still do not know who did this or why, and people shouldn’t jump to conclusions before we have all the facts, but make no mistake, we will get to the bottom of this,” Obama said from the White House several hours after the blasts. “Any responsible individuals, any responsible groups will feel the full weight of justice.”

He pledged the full help of the federal government and said: “The American people will say a prayer for Boston tonight.”

Suspicious packages were found after the blasts at three Boston subway stops, and authorities were investigating. New York police deployed extra security to landmarks, Pennsylvania Avenue in front of the White House was closed to foot traffic, and the Pentagon tightened security. Federal authorities briefly grounded flights at the Boston airport as a precaution.

The race is a signature event in Boston and has been run since 1897 on Patriots Day, the third Monday in April. Tens of thousands of spectators turn out each year to watch.

Race organizers said that almost 27,000 runners competed, representing 96 countries. The winners were Lelisa Desisa of Ethiopia for the men and Rita Jeptoo of Kenya for the women. A special marker at the 26th mile of the course, yards from the finish, had been set up to honor the 26 dead in the Newtown, Conn., elementary school shooting last December.

The elite men began running at 10 a.m., and the explosions were reported just before 3 p.m. The winners had long ago completed the race — Desisa finished with a time of just over 2 hours, 10 minutes — but the explosions came as masses of other runners were approaching the finish. NBC affiliate WHDH said that storefront windows nearby were blown out.

“Right now I’m in my condo with about 50-60 people I picked up off the street including marathon runners. Setting up a camp,” Corey Griffin told NBC News. “They have nowhere to go because everything is shut down. Officials said to get inside. This is crazy.”

Evan Kohlmann, a terrorism analyst for NBC News, said that authorities would probably examine residue from the blasts to determine their type.

Adding that it was premature to identify a culprit, he said: “If this was a deliberate act, unfortunately it certainly would reflect something that we’re seeing. There’s an emphasis on these soft-targeted attacks now. We’re moving away from the spectacular attacks and we’re moving into the small-grade, homegrown attacks.”

Will Ritter, spokesman for Massachusetts Senate candidate Gabriel Gomez, who was running the race, told NBC News that he heard what sounded like two explosions and saw smoke rising near the Boston Public Library. He said that he saw three fire engines and police running to the site.

“We heard two really large explosions in rapid succession, about a second apart from each other,” Ritter said. “Everybody kind of ducked and hit the ground.”

The AP reported that runners and race organizers were crying as they fled the scene, and that bloody spectators were carried to medical tents intended for exhausted runners. Runners who were still on the 26.2-mile course were being stopped and directed elsewhere, the Massachusetts Emergency Management Agency said. The agency suggested that people trying to reach loved ones use text messaging because of crowded phone lines.

Authorities gave a phone number for people in search of loved ones — 617-635-4520. They encouraged people with information about the blasts to call 1-800-494-TIPS.

Tracy Connor of NBC News contributed to this report. Reuters also contributed to this report.